This invention relates to coatings of the type used to protect components exposed to high temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to protective coatings that are capable of significantly contributing to the structural properties of the components they protect.
Certain components of the turbine, combustor and augmentor sections susceptible to damage by oxidation and hot corrosion attack are typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what may be termed a TBC system. Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic whose aluminum content provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) provides protection from oxidation and hot corrosion, and in the case of a bond coat promotes a chemical bond with the TBC. However, a thermal expansion mismatch exists between metallic bond coats, their alumina scale and the overlying ceramic TBC, and peeling stresses generated by this mismatch gradually increase over time to the point where TBC spallation can occur as a result of cracks that form at the interface between the bond coat and alumina scale or the interface between the alumina scale and TBC. More particularly, coating system performance and life have been determined to be dependent on factors that include stresses arising from the growth of the TGO on the bond coat, stresses due to the thermal expansion mismatch between the ceramic TBC and the metallic bond coat, the fracture resistance of the TGO interface (affected by segregation of impurities, roughness, oxide type and others), and time-dependent and time-independent plastic deformation of the bond coat that leads to rumpling of the bond coat/TGO interface. As such, advancements in TBC coating system have been concerned in part with delaying the first instance of oxide spallation, which in turn is influenced by the above strength-related factors.
Environmental coatings and TBC bond coats in wide use include alloys such as MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and diffusion coatings that contain aluminum intermetallics, predominantly beta-phase nickel aluminide and platinum-modified nickel aluminides (PtAl). In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions containing intermetallic phases, the NiAl beta phase is an intermetallic compound present within nickel-aluminum compositions containing about 25 to about 60 atomic percent aluminum. Because TBC life depends not only on the environmental resistance but also the strength of its bond coat, bond coats capable of exhibiting higher strength have been developed, notable examples of which include beta-phase NiAl overlay coatings (as opposed to diffusion coatings) disclosed in commonly-assigned U.S. Pat. No. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, U.S. Pat. No. 6,291,084 to Darolia et al., U.S. Pat. No. 6,620,524 to Pfaendtner et al., and U.S. Pat. No. 6,682,827 to Darolia et al. These intermetallic overlay coatings, which preferably contain a reactive element (such as zirconium and/or hafnium) and/or other alloying constituents (such as chromium), have been shown to improve the adhesion and spallation resistance of a ceramic TBC. The presence of reactive elements such as zirconium and hafnium in beta-phase NiAl overlay coatings has been shown to improve environmental resistance as well as strengthen the coating, primarily by solid solution strengthening of the beta-phase NiAl matrix.
In addition to the above, the suitability of environmental coatings and TBC bond coats formed of NiAlPt to contain both gamma phase (γ-Ni) and gamma-prime phase (γ′-Ni3Al) is reported in U.S. Patent Application Publication No. 2004/0229075 to Gleeson et al. The NiAlPt compositions evaluated by Gleeson et al. contained less than about 23 atomic percent (about 9 weight percent or less) aluminum, between about 10 and 30 atomic percent (about 28 to 63 weight percent) platinum, and optionally limited additions of reactive elements.
Aside from use as additives in MCrAlX overlay coatings and diffusion coatings, and as major constituents in intermetallic overlay coatings such as Gleeson et al., platinum and other platinum group metals (PGM) such as rhodium and palladium have been considered as a replacement for traditional bond coats. For example, commonly-assigned U.S. Pat. No. 5,427,866 to Nagaraj et al. discloses deposition of a thin protective layer (up to about 0.001 inch (about 25 micrometers)) of platinum, rhodium, or palladium on a substrate, diffusing at least a portion of the protective layer into the substrate, and then depositing a ceramic layer directly on the diffused protective layer. According to Nagaraj et al., elimination of a traditional bond coat reduces the weight of the coated article and reduces the likelihood of a detrimental secondary reaction zone (SRZ) forming in the substrate surface.
Though having the above-noted benefits, there are drawbacks to the use of environmental coatings and bond coats. For example, the maximum design temperature of a coated component is typically limited by the maximum allowable temperature of its environmental coating or bond coat (in the event of TBC spallation). A low melting point zone also tends to form between such coatings and their underlying superalloy substrate, further limiting the high temperature capability of the component. Another drawback is that the materials used to form environmental coatings and bond coats are relatively weak compared to the nickel and cobalt-base superalloys that form the components they protect. As a result, these coatings are considered dead weight that must be supported by the superalloy substrate, which is particularly detrimental to rotating airfoil applications such as turbine blades where the effect is greatly multiplied by the high G-field under which such components operate. As a result, airfoil components must be designed to be sufficiently strong to carry the weight of the coatings, often incurring yet additional weight penalty.
In view of the above, even with the existing advancements in materials and processes for environmental coatings and bond coats, there is a considerable ongoing effort to develop improved environmental coatings and TBC systems.